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To my constant surprise the issue of Pluto's planetary status -- which I think should be a dry technical issue -- really gets people riled. But it's also been my experience that the people who most want Pluto to be a planet know the least about it and the history of its discovery. So, I hope that this video can help correct that a little bit.

Just for the record, even though I imply (but don't directly say) that Eris is in the Kuiper Belt, I'm aware that it's actually further out and is normally classified as a Scatter Belt Object, but I decided to use the term Kuiper Belt in its most inclusive form for two reasons:

1) Simplicity. The distinction between resonant, classical and scattered objects is a lot of detail for a video intended as an introduction to the topic. (There will be a followup video covering planetary definitions in more detail in the coming weeks) People often start their understanding of something new by comparing it to something old. So both for historical and pedagogical reasons, comparing the new Kuiper Belt to the old Asteroid Belt seems the way to go when explaining Pluto's status for the first time.

According to the Minor Planet Center, which officially catalogues all trans-Neptunian objects, a [Kuiper Belt Object], strictly speaking, is any object that orbits exclusively within the defined Kuiper belt region regardless of origin or composition. Objects found outside the belt are classed as scattered objects. However, in some scientific circles the term "Kuiper belt object" has become synonymous with any icy planetoid native to the outer solar system believed to have been part of that initial class, even if its orbit during the bulk of solar system history has been beyond the Kuiper belt (e.g. in the scattered disc region). They often describe scattered disc objects as "scattered Kuiper belt objects.... A consensus among astronomers as to the precise definition of the Kuiper belt has yet to be reached, and this issue remains unresolved.

(Emphasis added)

So, even among astronomers, Eris is often referred to as part of the Kuiper Belt even though it would be more technically correct to group it with the scattered disc objects beyond the Kuiper belt.

On a completely unrelated note NASA made me LOL with their choice of title for this image:

Script:

Pluto: Planet or not?

Before we can answer this question we need to know what the word planet is for, and that takes us back to the ancient greeks who called Mercury, Venus, Mars, Jupiter, Saturn, the Moon and sun planets. Basically if it moved across the sky and was bright, it was a planet. This is a terrible start for the word because, 1) it excludes Earth from the list and 2) it groups wildly different things together.

But the greeks couldn't know how different the Moon was from Saturn, because the best technology they had to observe the Universe was sadly limited.

It would take several thousand years until the industrious Dutch made the first telescopes and astronomy became much more interesting.

Astronomers could now confidently rearrange the solar system -- an elegant scientific advance that no one could possibly object to -- and reclassify its parts, dropping the Sun and moon from the list of planets and adding Earth.

Now, if it orbited the Sun, it was a planet.

As time went on and telescopes got better and better each new century brought with it the discovery of a new planet.

Which brings us to this familiar solar system: nine planets orbiting one star.

And looking at this model makes people wonder, why do astronomers want to ditch Pluto?

The problem is pictures like this in textbooks are lies. Well, not lies exactly, but unhelpful. They give the impression that the planets are similar-ash in size and evenly-ish spaced, but the reality couldn't be more different.

Here, dear Terrans, is our home planet Earth, and this is Jupiter next to it at the correct scale -- rather bigger than you probably thought. If we take this diagram and adjust for the correct sizes of the planets it looks like this. Unless you're watching the video in fullscreen HD mode, you might not even be able to see Pluto.

So size differences are vast, and Pluto is the smallest by far. But it's not just small for a planet, it's also smaller than seven moons: Triton, Europa, our own Moon, Io, Callisto, Titan, and Ganymede.

Even if you show the correct relative sizes the distances are still a problem. Think about it, if Jupiter was this close to Earth it wouldn't look like a dot in the night's sky but would be rather overwhelming -- so it must be really far away, which makes drawing it to scale rather a challenge. If you want the length of a piece of paper to represent the distance from Mercury to Pluto, then giant Jupiter would be the size of a dust mite on that page, and Pluto a bacterium.

But excluding Pluto from the plant club just for being tiny and far away isn't reason enough and quickly brings out the Pluto defenders.

In order to understand what Pluto really is, we need to first discuss a planet you've never heard of: Ceres.

Back in the 1801, astronomers found a new planet in the huge gap between Mars and Jupiter -- it was a small planet, but they loved it anyway and named it Ceres.

The next year astronomers found another small planet in the same area and named it Pallas. A few years later they found a third one, Juno, and then, funnily enough, a fourth one, Vesta. And for a several decades children learned the 11 planets of the solar system.

But, astronomers kept finding more and more of these objects and became increasingly uncomfortable calling them planets because they were much more like each other than planets the on either side, so a new category was born: asteroids in the asteroid belt -- and the tiny planets were relabeled which is why you've never heard of them. And it was a good decision too, as astronomers have now found hundreds of thousands of asteroids, which would be a lot for a kid to memorize if they were all still planets.

Back to Pluto. It was discovered in 1930 making it the 9th planet. First estimates put Pluto about the size of Neptune, but with more observations that was revised down, and down and down. While Pluto shrank astronomers started to find other, similar objects orbiting in the same zone.

Sound familiar?

While school kids kept memorizing the nine planets, some astronomers grew uneasy about including Pluto because the size estimates continued to shrink, they learned that Pluto is made mostly ice, and they continued to find lots and lots of icy objects at the edge of the solar system just like Pluto.

This problem could be ignored as long as no one found an ice ball bigger than Pluto, which is exactly what happened in 2006 with the discovery of Eris. Once again, astronomers recategorized the solar system and grouped these distant objects, including Pluto, into a new area called Kuiper belt.

And that's the story of Pluto -- a miscategorized planted that finally found its home -- just like Ceres. But this story is really less about Pluto than it is about realizing the word 'planet' isn't very helpful.

The first four planets are nothing at all like the next four, so it's even a little weird to group these eight together which is why they often aren't and are separated into terrestrial planets and gas giants.

And now that we have telescopes that can see planets around stars not our own, and we've found rogue planets drifting in empty space and brown dwarfs -- objects that blur the very line between planet and star -- the word planet becomes even less clear.

So as we increase our knowledge of the Universe the category of 'planet' will probably continue to evolve, or possibly, fall out of favor entirely.

But, for the time being the best way to categorize the stuff in our solar system is into one star, eight planets, four terrestrial, four gas giants, the asteroid belt, and the distant Kuiper belt, home to Pluto.

Corrections:

1:32 Pluto is smaller than the seven moons listed, not nine as stated.